There is a great interest and need for developing compact, reliable high power mid-infrared radiation (mid-IR) sources for many types of applications, such as, spectroscopy, environmental monitoring, gas and chemical sensing and manufacturing process control. The most serious limitations to practical mid-IR sources has been the complexity of the near-IR pump source required and the cost, stability and reliability of nonlinear mixing mediums. However, nonlinear frequency mixing of high power near-IR laser diode sources would be an attractive technique for generating broadly tunable coherent mid-IR frequencies, such as in the range of about 2.0 .mu.m to 5.0 .mu.m.
The use of high power laser diodes for frequency conversion have been contemplated for sum or difference frequency mixing (SFM or DFM) applications, such as shown in FIG. 6 of U.S. Pat. No. 5,321,718, which is commonly owned by the assignee of this application and is incorporated herein by reference thereto. Disclosed are a pair of high tunable power laser diodes tuned to emit radiation of different wavelengths .lambda..sub.1, .lambda..sub.2, the beams of which are subsequently combined and provided as input to a nonlinear crystal device for difference frequency mixing producing a mid-infrared output of wavelength .lambda..sub.3 =.lambda..sub.1.multidot..lambda..sub.2 /(.lambda..sub.1 -.lambda..sub.2) or for sum frequency mixing producing blue or green light output of wavelength .lambda..sub.3 =.sub.1.multidot..lambda..sub.2 /(.lambda..sub.1 +.lambda..sub.2).
Presently, there are no reliable room temperature operated, broadly tunable mid-IR sources that are sufficiently compact to provide for an output over broad mid-range infrared radiation. No direct room temperature laser diode sources are available at wavelengths longer than about 2.4 .mu.m. Other sources, for example, employ comparatively large, higher than room temperature operated lasers, such as a Ar.sup.+ laser pumping a Ti:sapphire, YAG laser or a dye laser. Moreover, nonlinear frequency mixing to achieve desired long wavelength frequencies requires better frequency conversion performance in presently available materials. The existing nonlinear frequency mixing (NFM) techniques employed for frequency doubling or extending are quasi-phase matching (QPM) with difference frequency mixing (DFM) and optical parametric oscillation (OPO).
So far, mid-IR generation by difference frequency mixing (DFM) has been demonstrated using laser diodes, in lieu of gas lasers, with a AgGaS.sub.2 and AgGaSe.sub.2 material utilizing noncritical birefringent phase matching (BPM) and, also, by quasi-phase matching (QPM) using LiNbO.sub.3 waveguide material. An example of the former is found in U. Simon et al., "Difference Frequency Generation in AgGaS.sub.2 by Use of Single Mode Diode Laser Pump Sources", Optics Letters, Vol. 18(13), pp. 1062-1064 (Jul. 1, 1993), which involved the feasibility of using two single mode laser diodes as pump sources mixed in AgGaS.sub.2 crystal generating about 3 nW of low end, mid-IR power at about 2 .mu.m. Tunability is suggested but is limited to tuning by means of varying the temperature and current of the diodes by permitting their emission wavelength to be tuned over its narrow bandwidth, e.g., 2 nm which would correspond to about a range of 18 nm tuning of the mid-IR wavelength from the AgGaS.sub.2 crystal (page 1063, col. 1, first full paragraph). Moreover, it has been proposed by U. Simon et al. in the article, "Difference frequency Mixing in AgGaS.sub.2 by Use of a High Power GaAlAs Tapered Semiconductor Amplifier at 860 nm", Optics Letters, Vol. 18(22), pp. 1931-1933 (Nov. 15, 1993) to provide one of the sources, the signal or injection source, as a single mode, master laser diode pumping a semiconductor flared amplifier to enhance the pumping power output to about 1.5 W as input, together with the output from a Ti:sapphire laser as the pumping wave to the QPM DFM AgGaS.sub.2 crystal.
As for the employment of LiNbO.sub.3 waveguides, the article of L. Goldberg et al., "Difference Frequency Generation of Tunable Mid-IR in Bulk Periodically Poled LiNbO.sub.3 ", Advance Solid-State Lasers, (Post-deadline paper), pp. PD23-1 to PD23-3 (Jan. 30, 1995) and L. Goldberg et al., "Widely Tunable Difference Frequency Generation in QPM-LiNbO.sub.3 ", Paper CPD49, Conference on Laser and Electro-Optics (CLEO May, 1995), discloses a tunable QPM DFM in a bulk periodically poled LiNbO.sub.3 using Nd:YAG and Ti: sapphire laser as pumping sources with the DFM wavelength tuned over a range of about 3.0 .mu.m to 4.0 .mu.m by tuning the Ti: sapphire laser or rotating the QPM LiNbO.sub.3 crystal to change the effective grating period. The disadvantage of the foregoing systems for achieving mid-IR generation is the lack of room temperature, mid-IR tunability over a large expansive mid-IR range of 2.0 .mu.m to 5.0 .mu.m and lack of a delivery system for mid-IR generation that is compact in size, less costly and not requiring large size solid state and gas lasers operating at elevated temperatures requiring large operating powers.
LiNbO.sub.3 waveguide material is a robust, relatively inexpensive, highly reliable nonlinear material for DFM, and QPM permits the material to be tailored to mix arbitrary laser diode wavelengths through the choice of an appropriate ferroelectric domain reversal period or periodic poling. Recent advances in electric filed poling of LiNbO3 waveguide material allows QPM to be implemented in bulk interactions and has been employed in demonstrating mid-IR optical parametric oscillation (OPO). See L. E. Myers et al., "Quasi-Phase Matched 1.064 .mu.m-Pumped Optical Parametric Oscillator in Bulk Periodically Poled LiNbO.sub.3 ", Optics Letters, Vol. 30(1), pp. 52-54 (Jan. 1, 1995) and W. K. Burns et al., "Second Harmonic Generation in Field Poled, Quasi-Phase Matched, Bulk LiNbO.sub.3 ", IEEE Photonics Technology Letters, Vol. 6(2), pp. 252-254 (February, 1994). However, mid-IR generation has been demonstrated in quasi-phase matching (QPM) crystals relative to such laser diode sources only for relatively shorter wavelength applications and not reliably extended to more desired mid-IR wavelength applications. Moreover, as applied to AgGaS.sub.2 and AgGaSe.sub.2 crystals, the full spectral range of mid-IR generation cannot be easily derived because the input wavelength range required to cover the full 2.0 .mu.m to 5.0 .mu.m mid-IR range is very broad for noncritical phase matching.
What is needed is a mid-IR generating system providing for noncritical phase matching over a wide range of near-IR input wavelengths at room temperature that can be frequency mixed to selectively provide a desired mid-IR wavelength over the entire mid-IR range from about 2.5 .mu.m to 5 .mu.m.
An object of this invention is the provision for mid-IR frequency conversion over a wide frequency range.
Another object of this invention is the provision for high power, semiconductor/optical amplifier/laser sources as applied to nonlinear frequency mixing devices, e.g., high power laser diodes, master oscillator power amplifier (MOPA) devices, high power fiber amplifier or laser, laser or amplifier devices, Raman or Brillouin amplifiers or lasers, and rare earth single mode fiber amplifiers and lasers.
Another object of this invention is the provision of the use of near-IR laser diode sources to generate broadly tunable coherent mid-IR radiation, such as tunable across the mid-IR range of 2 .mu.m to 5 .mu.m.
Another object of this invention is the provision of a compact, room temperature, broadly tunable coherent source for selectively tuning a desired mid-IR wavelength.
Another object of this invention is the provision of high efficiency buried non-linear waveguide structures using quasi-phase matching (QPM) to convert developed near-IR power from room temperature laser sources to mid-IR power for use, for example, in a large spectrum of analysis, sensing and monitoring applications.